In recent years, the motors that drive the fans of outdoor units of air conditioners (hereinbelow, abbreviated as outdoor fans) have been controlled by inverters, wherein rotational speed is controlled in accordance with instructions regardless of the external load's size. Because outdoor fans are generally disposed outdoors, they are easily subject to loads in forward or reverse rotational direction caused by natural wind. As a result, motor current increases owing to loads in the reverse rotational direction, and the fan rotates at a speed that exceeds its target rotational speed owing to loads in the forward rotational direction; therefore, problems such as overcurrents, overvoltages, and loss of synchronization can sometimes arise and cause the fan to stop abnormally. One way of solving this problem is a method that has already been disclosed (e.g., refer to Japanese Patent Application Publication No. H8-303386), wherein a permissible rotational speed range, which is a speed range that is acceptable even when a motor is energized, is preset, and when the fan rotates at a speed outside of that permissible rotational speed, the motor is not energized.